Line scanning in a display

ABSTRACT

The present invention relates to a method of scanning lines in a display, a device for scanning lines and a portable electronic device including such a device. Driving luminance information for each pixel within a frame is divided into subfields ( 90, 92, 94, 96, 98, 100 ). Subfields are selected for scanning in a set of scanning cycles (Scan  0,  Scan  1,  Scan  2,  Scan  3,  Scan  4,  Scan  5 ) equivalent to the existing number of subfields, lines are scanned consecutively and selection of subfield is varied from line to line in each scanning cycle such that no two consecutive line scans use the same subfield and no line is scanned using the same subfield twice during the set. Image flicker caused by the subfields is reduced.

The present invention is directed towards a method and a device forscanning lines in a display as well as an electronic device includingsuch a device. More specifically the invention is directed towardsselection of luminance information for scanning lines in displays.

Displays are generally driven by using a field associated with eachpixel, where the field provides a luminance value that is providedduring a frame. In many digitally driven displays, this field is dividedinto smaller sections or subfields, often in order to be able to providea finer resolution of luminance information. In some applications thevarious subfields represent a different quantity of luminance. When suchsubfields are driven sequentially i.e. first one subfield is driven in ascanning cycle line by line followed by a next subfield line by lineuntil all subfields have been driven within the frame, visual imageflicker may occur. This visual image flicker may occur because of thediffering lengths of the subfields and the fact that they are drivensequentially at a specific rate. This flicker becomes less visible whenthe display rate is increased. However, it cannot be increased too muchbecause of the minimum addressing time of the different subfields.Increased display rates also leads to a higher power consumption, whichoften is not desirable when using the display driving scheme in aportable device.

U.S. Pat. No. 6,094,243 describes two different display driving schemesfor a liquid crystal display, pulse width modulation and frame ratecontrol. The document also describes subfields having different lengths.Flicker is in this document avoided by varying the voltage applied fromsubfield to subfield. The document therefore describes reduction of thesubframe periods of the most significant bits through driving them withhigher voltages. There is no solution mentioned to the problem offlicker due to the sequential driving of subfields.

There is thus a need for reducing flicker because of luminance artifactswithout having to increase the display rate and voltage.

The present invention is therefore directed towards solving theabove-mentioned problems associated with flickering caused by subfieldswithout increasing the display rate and voltage.

One object of the present invention is thus to provide a method ofscanning lines, which reduces the flickering associated with subfieldswithout raising the display rate and voltage.

According to a first aspect of the present invention, this object isachieved by a method of scanning lines in a display within a frame,where driving luminance information provided to the display for eachpixel within the frame is divided into subfields. The method includesthe steps of: selecting subfields to be used when scanning lines in aset of scanning cycles equivalent to the number of subfields existingfor driving the pixels, scanning the lines consecutively for the set ofscanning cycles, and varying the selection of subfield from line to linein each scanning cycle such that no two consecutive line scans use thesame subfield and no line is scanned using the same subfield twiceduring the set of scanning cycles, such that image flicker caused by thesubfields is reduced.

Another object of the present invention is to provide a device forscanning a display, which reduces the flickering associated withsubfields without raising the display rate and voltage.

According to a second aspect of the present invention, this object isachieved by a device for scanning a number of lines in a display withina frame using luminance values within a frame and comprising: at leastone conversion unit for converting received luminance values intodriving luminance information including subfields, and supplying thesubfields to a line driving unit, a line driving unit arranged to scaneach line consecutively with the luminance information of each pixel onthe display in a number of scanning cycles equivalent to the number ofsubfields existing for driving the pixels, and a control unit arrangedto provide variation of the selection of subfield from line to line foreach scanning cycle such that no two consecutive line scans use the samesubfield and no line is scanned using the same subfield twice during theset of scanning cycles, such that image flicker caused by the differentsizes of the subfields is reduced.

Yet another object of the present invention is to provide a portableelectronic device having a display and which reduces the flickeringassociated with subfields without raising the display rate.

According to a third aspect of the invention, this object is achieved bya portable electronic device comprising: a display, at least oneconversion unit for converting received luminance values into drivingluminance information including subfields and supplying the subfields toa line driving unit, a line driving unit arranged to scan each lineconsecutively with the luminance information of each pixel on thedisplay in a number of scanning cycles equivalent to the number ofsubfields existing for driving the pixels, and a control unit arrangedto provide variation of the selection of subfield

from line to line for each scanning cycle such that no two consecutiveline scans use the same subfield and no line is scanned using the samesubfield twice during the set of scanning cycles, such that imageflicker caused by the different sizes of the subfields is reduced.

Claims 3 and 10 are directed towards one variation of the invention,where subfields are provided in staggered order, i.e. consecutively fromline to line within a scanning cycle.

Claims 4 and 11 are directed towards another variation of the invention,where a complete random selection is made of subfields form line toline.

With the present invention flickering occurring because of artifactsassociated with a certain subfield from line to line and because of tothe length of a subfield is reduced. Since the display rate and voltageis not raised for accomplishing this, the invention is also powerefficient.

The basic idea of the invention is to provide variation of the ordersubfields are provided to a display from line to line in a consecutiveline-scanning scheme.

The expression line used here is intended to comprise lines in anydirection on the display and to comprise both rows and columns.

The above mentioned and other aspects of the invention will be apparentfrom and elucidated with reference to the embodiments describedhereinafter.

The present invention will be further described in relation to theaccompanying drawings, in which:

FIG. 1 shows a basic pulse width modulation driving scheme for adisplay,

FIG. 2 shows a basic frame rate driving scheme for a display,

FIG. 3 shows a basic frame length driving scheme for a display,

FIG. 4 shows a portable electronic device in the form of a cellularphone including a display for showing among other things videoinformation,

FIG. 5 shows a block schematic of a display unit according to theinvention connected to various image sources for driving a pixel in adisplay,

FIG. 6 shows a general frame length driving scheme for a display

FIG. 7 shows a variation of an enhanced pulse width modulation drivingscheme for a display,

FIG. 8 shows a standard frame length driving scheme in a number ofscanning cycles for a display,

FIG. 9 shows an enhanced frame length driving scheme according to theinvention in the same number of scanning cycles as in FIG. 8, fordriving a display according to the invention,

FIG. 10 shows the pulse width modulation scheme of FIG. 7 in a number ofscanning cycles for the display,

FIG. 11 shows an enhanced pulse width modulation driving schemeaccording to the invention in the same number of scanning cycles as inFIG. 10 for the display,

FIG. 12 shows a standard frame rate control driving scheme in a numberof scanning cycles for the display,

FIG. 13 shows an enhanced frame rate control driving scheme according tothe invention in the same number of scanning cycles as in FIG. 12 forthe display,

FIG. 14 shows a flow chart of a method of driving a display according tothe invention, and

FIG. 15 shows another flow chart for driving the display according to apreferred embodiment of the present invention.

Before describing the invention in more detail a few differentdisplay-driving schemes will be described in order to get a betterunderstanding of the invention. A lot of the discussion will also bemade in relation to gray level portrayal. It should however be realizedthat full color portrayal is applicable in line with the gray levelportrayal scheme to be described in the following. The principles of thegray level scheme is then applied for the colors red blue and green.

FIG. 1 shows a timing diagram for a basic gray level pulse widthmodulation (PWM) driving scheme for a display. In these types of schemesa pixel is driven with a root mean square (RMS) voltage during a certaintime of a frame period for a pixel. A luminance level supplied to thedisplay for the pixel then corresponds to the time the voltage isdriven. In FIG. 1 it is seen that a pixel is driven during a firstperiod 10 of the frame time T_(f) and not driven during a second period12 of the frame time. The timing is indicated with a thick black linehaving a high level during driving and a low level when a pixel is notdriven. In the figure the frame time is divided into 7 different timesections, corresponding to the resolution. The pixel is thus drivenduring 3 out of the seven time periods, which time period corresponds tothe luminance level provided. The duty cycle then determines theluminance value. This basic scheme provides one pulse for driving adisplay within a frame. There is thus one address cycle for a pixelduring the frame. A scanning cycle is indicated in FIG. 1 with an arrowat the beginning and end of the whole set of sections. Note that onlythe addressed line time is depicted in the figure.

FIG. 2 shows a basic gray level frame rate control (FRC) scheme during aframe T_(f). Here luminance information is provided in the form of threebits, bit 0, bit 1 and bit 2, which together provide eight gray scalevalues. In this scheme the frame is divided into 7 different subfieldsof equal length. Each subfield is driven once during a frame and istherefore covered by one address cycle, which is indicated with arrowsat both ends of each subfield. The subfields are according to this knownscheme scanned in order from left to right during a frame T_(f) for eachconsecutive scanning cycle. A subfield is either driven (on) or notdriven (off). A first subfield 14 corresponds to the bit 2 of luminanceinformation, a second subfield 16 corresponds to the bit 1 of luminanceinformation, a third subfield 18 also corresponds to the bit 2 ofluminance information, a fourth subfield 20 corresponds to the bit 0 ofluminance information, a fifth subfield 22 corresponds to the bit 2 ofluminance information, a sixth subfield 24 corresponds to the bit 1 ofluminance information and a seventh subfield 26 corresponds to the bit 1of luminance information. If for instance bit 2 is on, all subfieldshaving this information will be driven, whereas none of these will bedriven if the bit is off. Note that only the addressed line time isdepicted in the figure.

FIG. 3 shows a basic gray level frame length control (FRC) scheme inwhich there are only three subfields corresponding to three differentbits during a frame T_(f). Each subfield here corresponds to a bit andthe length of the subfield corresponds to the importance of the bit.This scheme is better described in EP application no. 02076071.6, whichis herein incorporated by reference. FIG. 3 thus shows a first subfieldhaving a certain length, a second subfield 30 having a longer length anda third subfield 32 having a third length, each provided during aseparate scanning cycle. Note that only the addressed line time isdepicted in the figure.

Each of these three driving schemes might give rise to flicker in adisplay, why the present invention is intended to solve this.

FIG. 6 shows a scheme during a frame T_(f), which is the frame lengthcontrol scheme with 6 binary weighted subfields capable of 2³*6 colors.For each pixel, the scheme comprises a first subfield 90, a secondsubfield 92 a third subfield 94, a fourth subfield 96, a fifth subfield98, and a sixth subfield 100. The temporal relationship between thesubfields can be based on for instance T/2^(n). In this way the 6-bitluminance values can be provided as luminance information in the form ofsubfields driving the display. Each subfield is scanned during ascanning cycle, which is indicated by thick arrows around each subfield.Note that only the addressed line time is depicted in the figure.

FIG. 7 shows a two section division for a PWM-driving scheme. Here afirst section 102 is driven followed by a second section 104. Thecorresponding scanning periods are shown with arrows. The drivensections can here be seen as subfields. However a larger black lineshows the actual driving of the sections within the subfields. In orderto minimize the switching between on and off states, the display isdriven during the latter part of the first section followed by theearlier part of the second section. Note that only the addressed linetime is depicted in the figure.

FIG. 4 shows a portable electronic device according to the invention inthe form of a cellular phone 34 having an antenna 38, a base band module40 and display 36. The portable electronic devices of today have moreand more advanced functions, one of them being video. With theseadvanced functions there is a need to display information on the displayof the phone like video information. It should however be understoodthat a cellular phone is just an example of one type of portableelectronic device where there is a need for better resolution in adisplay. The display is in the preferred embodiment a Color SuperTwisted Nematic Liquid Crystal Display (CSTN-LCD), although also othertypes can be used.

FIG. 5 shows a block schematic of a device for scanning 45, which isprovided in the phone of FIG. 4. The device 45 is connected to a videosource 42, like for instance an MPEG-4 video source, which delivers avideo stream or image data. The video source can in itself have receiveda video stream from a network to which the phone is connected. Thedevice 45 is also connected a data & graphics source 44, which deliversdata and graphics. These sources 42, 44 are connected to a videoprocessing unit 46 within the device 45. As can be seen from FIG. 5, thevideo source delivers so called 5-6-5 information, that is the colors tobe presented on the screen are coded with 5, 6, and 5 bits for red,green and blue, respectively. As is also clear from the figure the dataand graphics source delivers data with 3-3-2 resolution, which meansthat the video source delivers data of higher color resolution. Thesedifferent types of streams are then processed in the video processingunit 46, which converts the 3-3-2 stream from the data and graphicssource 44 to a 5-6-5 stream, by stuffing the least significant bits.This is only done in order to get uniform handling of different types ofdata. In the video processing unit there is also performed videoprocessing like gamma-correction. This is normally a non-linear functionx=y^(n), which converts video data to luminance values. Here n istypically 2.4. This function can be combined with the displaytransmission-curve compensation. Also dithering can be used in thisvideo processing unit.

The video processing unit 46 then submits the high-resolution luminancevalues (5-6-5) to a data conversion device 48, which converts thehigh-resolution luminance values to information suitable for driving thedisplay. In order to do this the data conversion device 48 includes aconversion unit 56 and a control unit 58 controlling the conversion.This information is then supplied to a frame memory 49, which iscontrolled by a timing and control subunit 50. The timing a controlsubunit 50 reads out luminance information from the frame memory 49 andsupplies these to a column driving unit 52 for driving the display 36.The timing and control subunit 50 also controls row drivers 54 tosequentially scan lines of the display. For each line scanned luminanceinformation is supplied to the column driving unit such that the display36 can be driven. The column and row driving units are thus connected tothe display 36 for driving it. Previously such driving has been donesuch that each section or subfield is provided within the same scan forall pixels from row to row. How it is done according to the inventionwill be described shortly.

When a visual artifact is linked to a specific subfield, it can appearon the complete display area during that period and will repeat at thedisplay rate. This may cause serious flicker artifacts at the frequencyused, especially when the display rate is low. When the display-rate isincreased the flicker will get less, however the dissipation willincrease and more power will also be needed for driving the display.

Besides this artifact, there might be another artifact. If the mostsignificant subfield is active for a rather long period, this may alsocause some artifacts. When the display-rate is increased this periodwill also get shorter and flicker will reduce.

FIG. 8 shows the driving of a display during a number of scan cycleswithin a frame in a frame length control scheme. The lines in FIG. 8 arenumbered from n to n+5, which is equal to the number of subfieldsexisting in the depicted scheme. This is only done for simplicity andbetter understanding of the problems existing. During a first scancycle, scan 0, all the lines are scanned sequentially with the firstsubfield 90. During a second scanning cycle, scan 1, all the lines arescanned sequentially with the second subfield 92. This is followed byscanning of the third subfield 94 during a third scanning cycle, scan 2,the fourth subfield 96 during a fourth scanning cycle, scan 3, the fifthsubfield 98 during a fifth scanning cycle and the sixth subfield 100during a sixth scanning cycle, which is the last scanning cycle in theframe. In the figure all the fifth 98 and sixth 100 subfields are notshown due to size limitations. It should however be realized that theyare scanned during the depicted scanning cycle. A visual artifactassociated with for instance the sixth subfield 100 can therefore bevery disturbing because of it being repeated for all lines within ascanning cycle.

The present invention reduces the influence from the previouslydescribed artifacts without having to raise the display rate. Thereforethe driving of the display according to a preferred embodiment of theinvention will now be described with reference to FIGS. 5, 9 and 15.FIG. 9 shows six different scanning cycles numbered from n to n+5, inwhich the order of the six subfields are varied from line to line in astaggered fashion. The method described will be limited to the scansmade within a frame. Only the lines n to n+5 are shown for each scan inFIG. 9, although the method is repeated for all lines of a display,which number is normally considerably larger. In this invention theenhanced frame length control scheme shown in FIG. 9 is used for drivinga pixel.

First the conversion unit 56 of the data conversion device 48 convertsluminance values into subfields according to the scheme depicted in FIG.6 under the control of control unit 58, step 126. The data conversiondevice 48 supplies these converted subfields to the frame memory 49 inan order such that they will be scanned in varying order from line toline, step 130. The control unit 58 thus selects the scanning order forthe subfields. For a preferred variation according to the invention, thesubfields for pixels in line n will then have the order: first 90,second 92, third 94, fourth 96, fifth 98 and sixth 100, the subfieldsfor pixels in line n+1 the order second 92, third 94, fourth 96, fifth98, sixth 100 and first 90, the subfields of pixels in line n+2 theorder: third 94, fourth 96, fifth 98, sixth 100, first 90 and second 92,the subfields of pixels in line n+3 the order: fourth 96, fifth 98,sixth 100, first 90, second 92 and third 94, the subfields of pixels inline n+4 the order: fifth 98, sixth 100, first 90, second 92, third 94and fourth 96 and the subfields of pixels in line n+5 the order: sixth100, first 90, second 92, third 94, fourth 96 and fifth 98. These abovedescribed orders are then repeated in the same fashion for allsubsequent lines. Thereafter the timing and control subunit 50 selectssubfields from the frame memory 49 in consecutive order and makes therow driving unit 54 scan the lines consecutively in all scanning cycles,step 132. This means that during a first scan, scan 0, all the subfieldswhich lie first in all the above described orders are scanned for eachline, followed by the subfields second in the order in scan 1 etc. untilscan 5 such that all information has been scanned in the frame.

FIG. 9 more clearly shows how this scanning is performed. In the schemethe first subfield 90 is chosen for the first line during a first scan,scan 0, the second subfield 92 is chosen for the next line n+1, thethird subfield 94 is chosen for the third line n+2, the fourth subfield96 is chosen for the fourth line n+3, the fifth subfield 98 is chosenfor the fifth line n+4 and the sixth subfield 100 is chosen for thesixth line n+5. This type of selection is then continued in the samefashion for all scanned lines within the display during the firstscanning cycle, scan 0. During the next scanning cycle, scan 1, thesecond subfield 92 is chosen for line n, the third subfield 94 is chosenfor line n+1, the fourth subfield 96 is chosen for line n+2, the fifthsubfield 98 is chosen for line n+3, the sixth subfield 100 is chosen forline n+4 and the first subfield 90 is chosen for line n+5 etc. for alllines of the display. The subfields are shifted one position for eachconsecutive scan up till scan 5. In this way all the information for thepixels are provided during a frame while at the same time shifting theselection of subfield from line to line. The subfields are thus selectedin a consecutive order from line to line.

An alternative way of performing the method is shown in FIG. 15. Here,the data conversion device 48 first converts the luminance values intom=six different subfields for each pixel of the display, step 106.Thereafter these subfields are all entered into the frame memory 49 inoriginal order. The timing and control subunit 50 then selects a firstline of the display to be scanned, step 108. After this the timing andcontrol subunit 50 sets a row counter N=m, i.e. to the number ofsubfields used, step 110, in order to define a set of lines within whichthe order of subfields may be varied. Thereafter the timing and controlsubunit 50 selects a first previously unselected subfield for all pixelswithin the selected line, step 112, and supplies this subfield to thecolumn driving unit 54 for a first line scan in a first scanning cycle,scan 0, step 114. For this first line n the first subfield 90 is drivenfor all pixels of the line. Then the line counter N is decreased by one,step 116. If the scanning cycle is ended, step 116, i.e. if the lastline of the display had been scanned during the scanning cycle, thetiming and control subunit 50 starts a new scanning cycle, scan 1, step120, returns to step 108 and selects a first line of this next scanningcycle, resets the line counter, step 110, and then continues with step112 and selects another subfield which has not previously been selectedfor the line, step 112, and scans the line while driving the selectedsubfield of all pixels in the line, step 114. If the scanning cycle wasnot ended, step 118, a further check is made if the line counter hasreached zero or not, step 120. If the line counter has reached zero,step 120, the next line is selected for scanning, step 124 and theprocess returns to step 110 and resets the line counter N, followed bynew selection of a subfield for the next line. If however the linecounter had not reached zero, step 120, the next line is selected andanother subfield not used for previously selected lines within the setof lines and not previously selected for this line during an earlierscan cycle is selected, step 122. Thereafter the line is scanned whiledriving the pixels with the selected subfield, step 114. In this way themethod continues until all lines have been scanned with varyingsubfields. The method thereafter continues in above described manner forconsecutive frames.

The above-described scheme mixes the subfield data from line to line. Inthis way each address scan will take the same period of time. The actualpictures show a mixture of all the subfields. Hence it will diffuse theartefacts of all the individual subfields and flicker effects. Stilleach pixel is driven with exactly the same signals in time as inordinary driving schemes. An extra advantage is that the columnswitching is more homogeneous and averaged over time, resulting in lesscross-talk effects.

The described methods can be varied, in that another order of selectioncan be provided. The selection can be provided as a complete randomselection with the limitations on selection set out in the flow chart ofFIG. 14, i.e. a subfield can only appear once for a line during a frameand the same subfield does not appear in consecutively selected lineswithin a scanning cycle. Different orders than staggered are alsopossible. One of the many variations is to drive the first subfield forthe first line, the third subfield for the second line, the fifthsubfield for the third line, the second subfield for the fourth line,the fourth subfield for the fifth line and the sixth subfield for thesixth line during scan 0 followed by the order second, fourth, sixth,first third and fifth subfields during scan 1 and third, fifth, first,fourth, sixth, second subfield during scan 2 etc. Yet another possiblevariation is the order first, sixth, second, fifth, third, fourthsubfields for scan 0 followed by the order fourth, first sixth, secondfifth, and third subfields for scan 1 etc. These are just a few of thecountless possible order variations that can exist. Naturally allpreviously described selection schemes can be controlled from either theconversion control unit or the timing and control subunit.

FIG. 10 shows a standard way of driving a display according to the pulsewidth modulation scheme. Because the subfield 101 is always provided inthe same scanning cycle, the same problems that were associated with thestandard frame length control scheme will occur. While only a singlescan needs to be driven, no frame memory is required. However the fieldrate will be very low and image flicker will occur. When increasing thefield rate, again a frame rate is required. The frame rate can beincreased by doubling the field data. However the gray scale resolutioncan be increased when a different time base is used for the repeatedaddress scan.

FIG. 11 shows a similar way of driving the display for the enhancedpulse width modulation scheme according to the invention for avoidingthe flickering. Here a first section 102 is driven for a first line nduring a first scan, scan 0. When the first section has been driven forthe required time of the length of the section, driving is ended andthen scanning for line n+1 is started. For line n+1 the actual drivingof the section is provided in the end of the time provided for thesecond section in order to reduce switching on and off the driving of aline. In this way the driving is continued for all lines of the displayduring the first scanning cycle. In the next scanning cycle, scan 1, theopposite driving is provided in that first the second section 104 isdriven for a length of time during the first part of the time providedfor the second section for line n followed by the driving of the firstsection 104 during a latter part of the time provided for this sectionin line n+1, etc. for all lines within the scan cycle.

This scheme interlaces the sections from line to line. In this way eachaddress scan will display a mixture of the two sections. Hence theartifacts of the individual sections will diffuse and flicker effectswill be reduced.

For each line the column data has only one transition per scan, whilethe modulated pulses are combined with the pulses of the next row. Thissaves column switching power.

For each line the duty-cycle of PWM can have 7 values (3 bits) resultingin 64 levels per pixel. The addressing scheme will provide a moreoptimal driving scheme when the weight of subfields have the fullaccuracy for each driven line, while no extra switching (dissipation) isrequired.

For a frame rate control scheme there are no visual artifacts associatedwith differing lengths of the subfields. There might still be someflickering though because of a visual artifact linked to a specificsubfield. FIG. 12 shows the standard way of driving the displayaccording to this scheme. For simplicity the figure only contains linesn to n+6, which is equal to the number of subfields existing in thedriving scheme. Here the display is first scanned with the firstsubfield 14 for all lines during a first scanning cycle, scan 0, thenscanned with the second subfield 16 for all lines during a secondscanning cycle, scan 1, thereafter scanned with the third subfield 18for all lines during a third scanning cycle, scan 2, then scanned withthe fourth subfield 20 for all lines during a fourth scanning cycle,scan 3, then scanned with the fifth subfield 22 for all lines during afifth scanning cycle, scan 4, then scanned with the sixth subfield 24for all lines during a sixth scanning cycle, scan 5, and finally scannedwith the seventh subfield 26 for all lines during a seventh scanningcycle, scan 6.

This flicker can be reduced with a variation of the scanning from lineto line according to the invention shown in FIG. 12. Here the subfieldshave been varied from line to line in the same manner as was done inFIG. 9. During the first scan, scan 0, line n thus scans the firstsubfield 14, line n+1 scans the second subfield 16, line n+2 scans thethird subfield 18, line n+3 scans the fourth subfield 20, line n+4 scansthe fifth subfield 24, line n+5 scans the sixth subfield 24 and line n+6scans the seventh subfield 26. During the second scan, scan 1, line nscans the seventh subfield 26, line n+1 scans the first subfield 14,line n+2 scans the second subfield 16, line n+3 scans the third subfield18, line n+4 scans the fourth subfield 20, line n+5 scans the fifthsubfield 22 and line n+6 scans the sixth subfield 24. This is continuedin the same manner as in FIG. 9 until the seventh scan, scan 6, whereline n scans the second subfield 16, line n+1 scans the third subfield18, line n+2 scans the fourth subfield 20, line n+3 scans the fifthsubfield 24, line n+4 scans the sixth subfield 24, line n+5 scans theseventh subfield 26 and line n+6 scans the first subfield 14. This alsothus reduces flickering. Note that the variations made to the framelength control scheme can of course also be made for this frame ratecontrol scheme. An extra advantage is that the column switching is morehomogenous and averaged over time, resulting in less cross-talk effects.

To improve on the trade-off between flicker and power, the scanfrequency can be adjusted to an optimum value.

With the described system for driving a display, the amount of flickeris reduced without having to increase the speed of scanning for a givenbasic scheme. The scanning scheme according to the invention reducesvisual image flicker, without the need for changing the display rate.While data of all subfields is displayed in a mixed way on the display,the visual image flicker, which can depend on a specific subfield, isspread over the frame-time. With this scheme a better image portrayal,low power consumption and feasible and cost-effective implementation isobtained.

When driving a LCD display module, the RMS voltage of the successivedriven subfields will determine the actual luminance of a pixel. Whenthe resulting luminance of the display is compared with the used drivingcodes, the actual transmission of the display can be characterised. Thedriving codes can be adjusted to get an optimal display performance.This characterisation needs to be done only once for a specific displaytype.

The control units can be implemented in the form of a microprocessorwith associated program memory.

The invention can be varied in some ways. The driving scheme can also beapplied with Multi Row Addressing (MRA), by grouping lines of the sameweight across repeating line-sets. The display can also be a separateentity, which is not included in the device for scanning lines. Thenumber of subfields used can also be varied in many ways.

The invention is furthermore not limited to cellular phones, but can beimplemented in any type of electronic device such as palmtops, laptopcomputers or electronic game machines.

In the description above lines have in all cases been rows of thedisplay. It is equally as well possible to scan columns and provideluminance information to row drivers instead.

1. Method of scanning lines in a display within a frame, where drivingluminance information provided to the display for each pixel within theframe is divided into subfields, the method including the steps of:selecting subfields to be used when scanning lines in a set of scanningcycles equivalent to the number of subfields existing for driving thepixels, scanning the lines consecutively for the set of scanning cycles,and varying the selection of subfield from line to line in each scanningcycle such that no two consecutive line scans use the same subfield andno line is scanned using the same subfield twice during the set ofscanning cycles, such that image flicker caused by the subfields isreduced.
 2. Method according to claim 1, wherein a scan of a lineincludes applying an RMS voltage corresponding to a value of thesubfield to a pixel.
 3. Method according to claim 1, wherein the step ofvarying includes selecting the subfields in a consecutive order fromline to line.
 4. Method according to claim 1 wherein the step of varyingincludes selecting the subfields in a random order from line to lineuntil all subfields have been selected and thereafter repeating therandom selection until all lines have been scanned.
 5. Method accordingto claim 1, wherein the subfields have varying lengths.
 6. Methodaccording to claim 1, wherein the subfields are subframes providedaccording to a frame length control scheme.
 7. Method according to claim1, wherein the subfields are subframes provided according to a framerate control scheme.
 8. Method according to claim 1, wherein thesubfields are provided according to a pulse width modulation scheme. 9.Method according to claim 1, wherein the subfields are providedaccording to a combination of schemes listed in claims 5, 6 and
 7. 10.Device for scanning a number of lines in a display within a frame usingluminance values within a frame and comprising: at least one conversionunit for converting received luminance values into driving luminanceinformation including subfields, and supplying the subfields to a linedriving unit, a line driving unit arranged to scan each lineconsecutively with the luminance information of each pixel on thedisplay in a number of scanning cycles equivalent to the number ofsubfields existing for driving the pixels, and a control unit arrangedto provide variation of the selection of subfield from line to line foreach scanning cycle such that no two consecutive line scans use the samesubfield and no line is scanned using the same subfield twice during theset of scanning cycles, such that image flicker caused by the differentsizes of the subfields is reduced.
 11. Device according to claim 10,wherein the control unit is arranged to select the subfields in aconsecutive order from line to line.
 12. Device according to claim 10,wherein the control unit is arranged to select the subfields in a randomorder from line to line until all subfields have been selected andthereafter to repeat the random selection until all lines have beenscanned.
 13. Device according to claim 10, wherein the subfields havediffering lengths.
 14. Device according to claim 10, wherein thesubfields are provided as subframes according to a frame length controlscheme.
 15. Device according to claim 10, wherein the subfields areprovided as subframes according to a frame rate control scheme. 16.Device according to claim 10, wherein the subfields are providedaccording to a pulse width modulation scheme.
 17. Device according toclaim 10, wherein the subfields are provided according to a combinationof schemes listed in claims 13, 14 and
 15. 18. Portable electronicdevice comprising: a display, at least one conversion unit forconverting received luminance values into driving luminance informationincluding subfields and supplying the subfields to a line driving unit,a line driving unit arranged to scan each line consecutively with theluminance information of each pixel on the display in a number ofscanning cycles equivalent to the number of subfields existing fordriving the pixels, and a control unit arranged to provide variation ofthe selection of subfield from line to line for each scanning cycle suchthat no two consecutive line scans use the same subfield and no line isscanned using the same subfield twice during the set of scanning cycles,such that image flicker caused by the different sizes of the subfieldsis reduced.